The present invention relates to a polarimeter in an all-fiber configuration, and more particularly to an assembly for determining the polarization, the degree of polarization and the power of light guided in a glass fiber, the use thereof, as well as a polarimetric method.
Light is an electromagnetic wave, the electric field strength components of which are oscillating with the optical angular frequency xcexa9 in the x-y plane orthogonal to the propagation direction z. Each wave may be separated into 2 orthogonal partial waves, the amplitudes and phase relationships of which uniquely describe the polarization.
In the case of linear partial waves:
E(t)=[Ex cos(xcexa9t+xcfx86x)Ey cos(xcexa9t+xcfx86y)][ex, ey]
A polarization variation is caused by a variation in the phase difference xcex94xcfx86=xcfx86yxe2x88x92xcfx86x or by a variation in the ratio of amplitudes.
For describing the polarization, several equivalent parameters are usual. Aside from the parameters of the polarization ellipse, azimuth xcex8 and ellipticity angle xcex5, the normalized Stokes parameters s1, s2, s3 are widespread. A complete description of even only partially polarized light waves gives the Stokes parameters S0, S1, S2 and S3. From these the normalized Stokes parameters s1, s2, s3 are derivable for describing the polarization state, the degree of polarization and the total power.
The refractive index n of a wave plate is direction-dependent. Therefore, the generally linear partial waves experience different phase velocities and obtain a phase difference.
A polarizer attenuates the partial wave in its blocking direction considerably more than the orthogonal component in its transmission direction. Therefore, the transmitted power becomes polarization-dependent and a simple detection of the polarization is realized.
The use of a polarimeter and a polarimetric method, respectively, has the following application fields:
Determining the degree of polarization (DOP)
Determining the degree of polarization (DOP) as a control signal in a polarization mode dispersion (PMD) compensator
Determining the polarization-dependent attenuation and loss (PDL), respectively, of optical fibers and components
Determining the polarization mode dispersion (PMD) of optical fibers and components
Analysis of birefringent and polarizing materials
Determining the extinction ratio (ER) in polarization maintaining fibers
Evaluation of sensors on a polarimetric basis (e.g. Faraday current sensor)
Extraction of control signals in automatic polarization controllers and many other things.
Aside from xe2x80x9ccomplete polarimetersxe2x80x9d, which detect all of the four Stokes parameters, there are means that determine only the deviation of the polarization state from a desired polarization state. This may be realized by simple polarizers, polarization beam splitters, etc.
The polarization of the light may be described mathematically by means of the Stokes vector. The Stokes vector is completely determined by the four Stokes parameters S0 . . . S3. The Stokes parameters are defined as follows: S0(absolute power), S1 (linearly horizontally polarized component less the linearly vertically polarized component), S2 (linearly 45xc2x0 polarized component less the linearly xe2x88x9245xc2x0 polarized component), S3 (right-handed circularly polarized component less the left-handed circularly polarized component).
For determining the polarization state, the degree of polarization and the power of the light, all four parameters of the Stokes vector have to be determined.
A polarimeter in the form of an assembly having a rotating wave plate in combination with a polarizer fixedly arranged in front of a detector is known. From the detected signal, the four Stokes parameters may be determined. However, the mechanically moving parts limit the measurement result speed.
There are also known various polarimeter assemblies, using beam splitters, polarization beam splitters, polarizers and wave plates, which separate the incident light beam such that the four Stokes parameters may be determined with at least four correspondingly disposed detectors. However, these assemblies normally require a high adjustment effort, see T. Pikaar et al.: Fast complete polarimeter for optical fibres; E-FOC 1989.
Another disadvantage of the assemblies mentioned above is the fact that with these assemblies an inline measurement, namely a determination of the polarization characteristics of the light guided in the glass fiber, usually is not possible. So-called fiber polarimeters or inline polarimeters avoid this disadvantage.
There are known various embodiments of fiber polarimeters. In the patent specification (U.S. Pat. No. 5,815,270) an assembly having a 1xc3x975 fusion coupler as well as succeeding polarizers and wave plates is disclosed.
Another known assembly is presented in R. M. A. Azzam: Inline light saving photopolarimeter and its fiber optic analog; Optic Letters, Vol. 12, No. 8, pp. 558-560, 1987 where polarization-dependent couplers are used for determining the Stokes parameters.
Another known assembly is presented in M. A. Habli: Experimental implementation of a fiber optic four detector photopolarimeter; Optik, Vol. 110, No. 9, pp. 433-435, 1999. There partially ground fibers are used to couple a polarization-dependent portion of the light out of the fiber.
The patent specification (U.S. Pat. No. 6,211,957B1) discloses another assembly of a fiber polarimeter. According to this, oblique fiber Bragg gratings are used, where the grating period and the angle between grating plane and fiber axis are selected such that light can couple from the guided fundamental mode into a radiation mode. This coupling is highly polarization-dependent. For determining the four Stokes parameters four differently oriented gratings are used where, in addition to the discrimination between right-handed circularly and left-handed circularly polarized light, a UV-induced wave plate is interposed. UV-induced birefringence is described in T. Erdogan et al.: Characterization of UV-induced birefringence in photo-sensitive Ge-doped silica optical fibers; J. Opt. Soc. Am. B/Vol. 11, No. 10, pp. 2100-2105, 1994. The generation of birefringence by bending the glass fiber is described in R. Ulrich et al.: Bending-induced birefringence in single-mode fibers; Optics Letters, Vol. 5, No. 6, June 1980.
This solution is disadvantageous in that the fiber Bragg gratings have to be inscribed with four different orientations to the fiber axis (0xc2x0, 90xc2x0 and 45xc2x0, 135xc2x0). In manufacturing the fiber Bragg gratings, this may be achieved by an appropriate rotation of the glass fiber about the fiber axis, but is associated with considerable effort. This also implies that the individual fiber Bragg gratings couple out light in four different directions. Thus, the use of planar detector rows or detector arrays is not possible.
Another disadvantage is the asymmetry of the coupled-out polarization components. With respect to the input, the polarization states linear 0xc2x0, linear 45xc2x0 and linear 90xc2x0 as well as a nearly circular polarization are coupled out. This assembly inevitably causes polarization-dependent losses (PDL) of the whole assembly, since the PDL of the individual gratings (components of the light intensity of a certain polarization direction are coupled out of the fiber) do not compensate for each other. Further, with three linear polarization states and an approximately circular polarization state, in the analysis of any polarization state the optimum cannot be achieved when real detector currents are evaluated.
What is desired is a technologically simpler and cheaper fiber polarimeter having better quality characteristics, the use thereof, as well as a corresponding polarimetric method.
According the present invention provides that fiber Bragg gratings are inscribed at a certain angle to a fiber axis such that at each grating two portions of the light, depending on the polarization, are coupled out of the fiber. This radiated light has two spatially separated intensity maximums occurring in different polarization states, which are detected or can be detected, respectively, by means of an appropriate number of detectors or preferably detector pairs. The manufacture of such fiber Bragg gratings is preferably performed by UV-induced refractive index modulation in the core of the glass fiber. In one embodiment, the interference pattern required for this is generated by means of an appropriate phase mask, wherein the phase mask is disposed at a certain angle to the fiber axis. Generally, a single-mode standard fiber is used. However, a glass fiber having a special refractive index and doping profile may be used to increase the coupling-out efficiency, to optimize the difference of the coupled-out polarization states, or to improve the extinction ratio (ER) of the polarization states thus being coupled out.
In one configuration of the assembly the fiber polarimeter has at least two special fiber Bragg gratings and an interposed wave plate. The fiber Bragg gratings may be differently oriented with respect to the fiber axis. In this assembly, preferably only two (instead of four) fiber Bragg gratings are required. The two powers coupled out by each grating, depending on the polarization, are each detected by one detector pair and converted to an electrical signal. For example, the two polarization states that a fiber Bragg grating couples out can be 0xc2x0 linearly polarized and 90xc2x0 linearly polarized. These signals are detected by appropriate hardware and further processed by software. From these data, by means of an algorithm and with regard to calibration data, the Stokes parameters are determined. Preferably photodiodes having an appropriate spectral sensitivity are used as detectors. The detectors, preferably detector pairs, are arranged such that their positions and sizes correspond to the position and size of the two spatially separated maximums of the coupled-out light. Since each fiber Bragg grating couples out two different polarization states and radiates in spatially different angles, the use of double photodiodes as the detectors is feasible.
In another embodiment of the assembly the detectors, or preferably the detector pairs, are disposed as close to the fiber as possible in order to achieve a signal intensity as high as possible. Another improvement of the signal intensity and the suppression of interfering Fabry-Perot effects may be achieved by avoiding the glass-air junction at the fiber surface and the beam spreading associated therewith because of the lens effect. The wave plates required in the assemblies may be produced by bending the glass fiber.
Further configurations are UV-induced birefringence, the use of a section of a birefringent glass fiber (polarization maintaining glass fiber), or the generation of birefringence by compressive loading the glass fiber. Generally, only a small portion of the light is coupled out by the fiber Bragg gratings, so that the insertion attenuation of the assembly is very low. But, since the coupled-out power is polarization-dependent, the assembly may have a low polarization-dependent loss (PDL). In individual gratings similarly tainted with PDL, the PDL is avoided by appropriate selection of the coupled-out polarization states and appropriate parameters for the wave plate. Of the totally four coupled-out polarization states, the polarization states coupled out by the same fiber Bragg grating respectively yield a polarization average value. If the average values of the coupled-out polarization states of the two fiber Bragg gratings are orthogonal to each other, then the polarization-dependent losses of the whole assembly compensate for each other and the whole assembly does not have a PDL.
Alternatively, a further fiber Bragg grating may be disposed succeeding each fiber Bragg grating of the assemblies, the parameters of which are identical, the orientation of which is orthogonal to the existing fiber Bragg grating, so that the polarization-dependent losses of one fiber Bragg grating are compensated for by a succeeding fiber Bragg grating.
In yet another embodiment of the assembly the gratings are inscribed in slightly birefringent (polarization maintaining) glass fiber. In this assembly the section of the fiber located between the gratings has the function of the wave plate.
Another embodiment results if the assembly is used for the generation of a control signal for the polarization-correct coupling into the principal axis of a polarization maintaining fiber. For this, the polarization maintaining fiber is disposed directly at the output, i.e. immediately after the last grating. In this variant, the orientation of the principal axes of the polarization maintaining fiber may be any with respect to the orientation of the gratings. Advantageously, the calibration of this assembly as a polarimeter is performed such that the polarization is measured with respect to the principal axes of the slightly birefringent (polarization maintaining) fiber.
In a further embodiment of the assembly for extraction of a control signal for the polarization-correct coupling into a polarization maintaining fiber, only one fiber Bragg grating is used, where the orientation of the two polarization states coupled out by the grating does not have to coincide exactly with one of the principal axes of the birefringent fiber. This fact allows for greater manufacturing tolerances in the angle adjustment between the principal axes of the polarization maintaining fiber and the coupled-out polarization states. From the two detector currents, a control signal may be generated, which provides a maximum output for the two coupled-out polarization states when coupling into the principal axis of the polarization maintaining fiber occurs. The achievable quality for the optimum coupling into the polarization maintaining fiber, expressed as the extinction ratio (ER), thereby may be substantially increased over conventional assemblies. For the operation of this assembly, it doesn""t matter whether the fiber Bragg grating is disposed immediately in front of the polarization maintaining fiber or in the same.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.